Many process control valves are actuated (e.g., pneumatically actuated) using well-known diaphragm type or piston type actuators. Actuators may be used to automate control valves such as, for example, linear valves, rotary valves, etc. by supplying force and motion to open or close a valve. Linear valves such as gate, globe, diaphragm, pinch, and angle valves typically have a valve stem (e.g., a sliding stem) that moves a flow control member (e.g., a valve plug) between an open position and a closed position. Rotary valves such as butterfly valves, ball valves, and disk valves typically have a valve shaft that moves a flow control member between an open position and a closed position. An actuator stem operatively couples a linear valve stem or a rotary valve shaft to the actuator (e.g., a pneumatic actuator, hydraulic actuator, etc.).
In operation, a positioner or control unit supplies a control fluid (e.g., air) to the actuator to cause the actuator to position the valve stem or shaft and, thus, the flow control member to a desired position to regulate fluid flow through a valve. When the valve is closed, the flow control member is typically configured to engage an annular or circumferential seal that encircles the flow path through the valve to prevent fluid flow (e.g., in one or both directions) through the valve.
In process control systems, it may be necessary to manually override the position of the flow control member to the open position, the closed position, or any other desired position. For example, it may be necessary to open a valve to prevent overpressurization of a vessel or it may be necessary to close a valve to prevent a spill (e.g., a chemical spill) during emergency situations, power failures, or if the control fluid (e.g., air) supply to an actuator (e.g., a pneumatic) is shut down. Some known example manual override mechanisms provide a hand wheel and screw configuration that is typically mounted directly to an actuator to manually operate a valve. However, these known manual override mechanisms limit manual operation of some valves in one direction and, thus, cannot be used to operate the valve in the other or opposite direction once these valves are manually operated. Therefore, these known manual override mechanisms are not suitable for use with double acting actuators (i.e., actuators in which pressurized fluid is used to open and close the valve).